On a busy server I'd NEVER use only one partition; though DA staff may disagree with me; they recommend otherwise.
I have no idea how much space you'll need for a /usr partition, because (though RHL doesn't enforce it) most people use /usr for their own software installations, and I have no idea how much software you'll eventually install on your server. I've never run out of space with 3G /usr partitions. Some RH documentation also recommends separate /usr and /usr local partitions; I've never seen a need for it, and I don't do it.
I currently use 10G /usr and /var partitions; it's likely going to be enough through the life of current servers, and with today's drive sizes, it's certainly available.
And you'll unlikely ever need more than 100 M for a /boot parittion; my most used and must updated systems have never gone above 25 M. I use 100 M.
While I won't fall into the trap of telling you what you should use, I can tell you that your first suggestion should work for my clients, though it's a bit overkill.
Note to others besides rldev reading this post: This post is NOT intended to make any representations about freeBSD installations of DA; I'm not familiar enough with freeBSD directory usage to be of any help in the freeBSD environment.
Jeff